When a magnetic field of not less than a lower critical magnetic field Hcl is applied to a superconductor, quantized flux lines (φ0=2.07×10−15 Wb) are formed and penetrate into the superconductor. When a current is caused to flow in this state, the Lorentz force acts on the quantized flux lines. When these quantized flux lines begin to move, a voltage is generated and the superconducting state is broken. It is known that, for example, in a superconducting film formed from a high-temperature oxide superconductor YBa2Cu3O7−x(YBCO), dot-like defects such as naturally introduced oxygen deficiency and fine Impurities function as pinning centers of quantized flux lines. Furthermore, it is known that one-dimensional defects such as dislocations and two-dimensional defects such as crystal grain boundaries function also as pinning centers. In this case of YBCO, it is important that these crystal defects be present perpendicular to the film plane. In general, YBCO-based high temperature superconductors are materials which have high crystal anisotropy and, therefore, when a magnetic field is applied parallel to the c-axis of a crystal, Jc tends to decrease greatly compared to a case where a magnetic field is applied perpendicularly to the c-axis. A usually used YBCO thin film is formed so that the c-axis is perpendicular to the film plane (surface), and, therefore, Jc decreases greatly when a magnetic field is applied perpendicularly to the film plane (surface). When a superconducting tape fabricated from a YBCO thin film is used to form a coil, magnetic field components of low Jc parallel to the c-axis govern coil properties, since a parallel magnetic field and a perpendicular magnetic field are applied to the tape. However, when one-dimensional defects or crystal grain boundaries are present in a direction parallel to the c-axis, they become pinning centers of quantized flux lines and Jc in this direction is improved. Therefore, the crystal orientation of one-dimensional defects or crystal grain boundaries is very important for improvement in coil properties. In contrast, this does not apply to dot-like defects etc. since they are isotropic.
The relationship between the dislocation density in a YBCO film and Jc has been reported by Dam (see B. Dam et al., Nature, Vol. 399, p 439, 1999). According to the report, dislocation densities of 10 μm−2 to 100 μm−2 can be obtained by changing film forming conditions in various ways and Jc increases with increasing dislocation density, although it is difficult to control the density per unit area of dislocations which are naturally introduced during film growth.
Crystal grain boundaries function not only as pinning centers, but also as barriers of superconducting currents. In fact, in a high temperature superconducting film of YBCO etc., Jc is very small in a grain boundary having a large inclination (the angle of a grain boundary to a normal line of the ab-plane of YBCO), but large Jc is maintained when the inclination is low. A low angle grain boundary can be regarded as a dislocation array. Although a dislocation is an insulator (non-superconductor), in a low angle grain boundary having a large spacing between dislocations, a strongly-coupled superconducting part exists between dislocations and a large superconducting current flows through the low angle grain boundary. However, when the inclination increases and the strains of dislocations begin to overlap, the current becomes less likely to flow. If boundary planes are parallel to a current flowing direction, they become very effective pinning centers. In general, however, boundary planes exist randomly, it is difficult to control Jc by controlling the inclination of boundary planes.
On the other hand, fine precipitates having a size close to the coherent length of the superconductor are also effective as pinning centers. Furthermore, artificial defects introduced by lithography and columnar crystal defects introduced by electron beam irradiation and heavy ion irradiation also become pinning centers. There is a possibility that desired pinning centers can be introduced by lithography in a film.
In a case where electron beam exposure is used, there is a report that the pin diameter can be decreased to the order of 10 to 20 nm, although it has not been able to reduce the pin diameter to the nano level. Also, the pin spacing can be adjusted to the same extent. Examples of measurement experiment of critical currents show that some peaks appear in superconducting properties in magnetic field depending on the relationship between quantized flux lines and pin arrangement (see J. Y. Lin et al., Phys. Rev. B54, R12712, 1996). Although this method is effective in artificial pin introduction, from a practical viewpoint, the throughput is low and the cost is too high for a large area fabrication and for wire fabrication. In heavy ion irradiation and the like, columnar defects are formed in superconducting crystals and this is effective in improving Jc. However, the equipment cost and the cost of ion acceleration are very high. Furthermore, in some cases materials are radioactivated and hence these methods are not practical.
In order to introduce crystal defects such as dislocations in a film, there is also available a method by which island-like crystals such as nano dots are formed on a substrate surface and a superconducting film is formed on the island-like crystals. There is an exemplary report that, in this case, Jc is improved by forming nano dots of Ag on a substrate (see A. Crisan et al., Appl. Phys. Lett., Vol. 79, p 4547, 2001). A literature of Dam suggests a principle that when fine precipitates exist in the process of growth of a film on a substrate, the continuity of film growth is lost on the fine precipitates, resulting in crystal defects, dislocations and grain boundaries (see B. Dam et al., Physica C341-348, p 2327, 2000). According to these techniques, however, the arrangement of introduced defects is random and the pinning force is averaged. Therefore, these techniques have their limits in drastically improving Jc.